【运维经】第44章——__builtin_expect 分支预测

运维经–目录

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__builtin_expect 分支预测

1.CPU流水线设计
CPU流水线设计，大大减少了取指令等待时间，但分支判断导致CPU执行跳转指令，降低了流水线的效率。
gcc引入了__builtin_expect，作用是允许程序员将最有可能执行的分支告诉编译器。

2.定义

/*
 * Using __builtin_constant_p(x) to ignore cases where the return
 * value is always the same.  This idea is taken from a similar patch
 * written by Daniel Walker.
 */
# ifndef likely
#  define likely(x)	(__branch_check__(x, 1, __builtin_constant_p(x)))
# endif
# ifndef unlikely
#  define unlikely(x)	(__branch_check__(x, 0, __builtin_constant_p(x)))
# endif

if(likely(value)) //等价于 if(value)
if(unlikely(value)) //也等价于 if(value)

3.例子

#define LIKELY(x) __builtin_expect(!!(x), 1)
#define UNLIKELY(x) __builtin_expect(!!(x), 0)
 
int test_likely(int x)
{
    if(LIKELY(x))
    {
        x = 5;
    }
    else
    {
        x = 6;
    }
    return x;
}
 
int test_unlikely(int x)
{
    if(UNLIKELY(x))
    {
        x = 5;
    }
    else
    {
        x = 6;
    }
    return x;
}

$ gcc  -fprofile-arcs -O2 -c like.c -o like.o
$ objdump -d  like.o

0000000000000000 <test_likely>:
   0:	48 83 05 00 00 00 00 	addq   $0x1,0x0(%rip)        # 8 <test_likely+0x8>
   7:	01 
   8:	85 ff                	test   %edi,%edi
   a:	b8 05 00 00 00       	mov    $0x5,%eax
   f:	74 07                	je     18 <test_likely+0x18> // 此处的效果是eax不为零时，不需要跳转。即x为真是不跳转。
  11:	f3 c3                	repz retq 
  13:	0f 1f 44 00 00       	nopl   0x0(%rax,%rax,1)
  18:	48 83 05 00 00 00 00 	addq   $0x1,0x0(%rip)        # 20 <test_likely+0x20>
  1f:	01 
  20:	b8 06 00 00 00       	mov    $0x6,%eax
  25:	c3                   	retq   
  26:	66 2e 0f 1f 84 00 00 	nopw   %cs:0x0(%rax,%rax,1)
  2d:	00 00 00 

0000000000000030 <test_unlikely>:
  30:	48 83 05 00 00 00 00 	addq   $0x1,0x0(%rip)        # 38 <test_unlikely+0x8>
  37:	01 
  38:	85 ff                	test   %edi,%edi
  3a:	75 14                	jne    50 <test_unlikely+0x20> // 此处的效果是edx为零时,不需跳转。即x为假时不跳转。
  3c:	48 83 05 00 00 00 00 	addq   $0x1,0x0(%rip)        # 44 <test_unlikely+0x14>
  43:	01 
  44:	b8 06 00 00 00       	mov    $0x6,%eax
  49:	c3                   	retq   
  4a:	66 0f 1f 44 00 00    	nopw   0x0(%rax,%rax,1)
  50:	b8 05 00 00 00       	mov    $0x5,%eax
  55:	c3                   	retq   

Disassembly of section .text.startup: